Some other thoughts - I always suggest to customers that they attempt to fix a problem starting w/the cheapest stuff first - things that probably are due to be replaced anyway, & won't be a waste of $$$$ if that doesn't solve the immediate problem.
How's your air filter? If using a paper element, I would want to replace it at least every 10K, although hardly anyone does. But these catch a lot of dirt over time, & I've seen many folks replace tons of expensive stuff, when the only problem turned out to be a dirty filter. BTW, as you may know, appearance doesn't tell you much - if one actually looks filthy, you've gone way too long!!! Running rich can cause the computer do do many strange things, & many other things can cause an engine to run rich - even a bad coolant temp sensor (the computer thinks the engine is always cold, hence always runs rich).
Also, have you used fuel system cleaner regularly - injectors clogged maybe? Or did you just run some which might have loosened up some crud?
Have you tested fuel pressure? That should help you to eliminate or pinpoint some possibilities.
Bad O2 sensors are another very common problem I see a lot of, too - will also cause engine to run rich, hesitation, surging.
O2 Sensor Info
